1. Field of the Invention
The present invention relates to viscosity measurement devices and, more particularly, to a piezoelectric device that determines the relative viscosity of a fluid whose viscosity is unknown.
2. Brief Description of the Prior Art
A fluid (liquid or gas) is a substance which undergoes continuous deformation when subjected to a sheer stress. When a fluid is set in motion, internal frictional forces act to oppose the motion of the fluid. This internal resistance to flow is known as viscosity. The theoretical viscosity of a fluid is defined as the force per unit area necessary to keep a plate, separated by a thin layer of fluid from a second plate, moving at a constant speed. If you measure the force required to keep the upper plate moving, you find that it is proportional to the area of the plates and to the ratio of the velocity of the upper plate to the distance separating the plates. This may be empirically written: ##EQU1## The constant of proportionality .eta. (the Greek letter eta) is called the coefficient of viscosity. Through manipulation of the above equation, it can be seen that viscosity is in units of mass/(time-length). In the c.g.s. system, where the basic units of measurement are the centimeter, gram, and second, the unit of viscosity is the poise, where one poise equals one dyne-second per square centimeter. Viscosities are usually tabulated, however, in centipoises, being 1/100th of a poise.
There are several alternative measurements for viscosity. The "relative viscosity" of a fluid is the ratio of its viscosity to that of water (at 68.degree. F.). Since the viscosity of water at 68.degree. F. is very nearly 1 centipoise, the relative viscosity of a fluid is generally equal to its viscosity in centipoises. The kinematic viscosity of a fluid is its viscosity divided by its density. The c.g.s. unit of kinematic viscosity is called the stoke and equals one square centimeter per second.
Viscosity is an important useful property for any activity involving liquid flow. The viscosity of a liquid falls as temperature increases, and determining the temperature dependance of the viscosity of a fluid may be essential in assessing the suitability of certain oils or fuels. For example, the viscosity of aviation oils and fuels is critical because they must function efficiently at sub-zero temperatures. Also, information concerning the molecular weight and shape of organic molecules can be obtained from determinations of viscosity.
Viscosity is presently measured by three common methods. Each of these methods compares the viscosity of fluids rather than actually measuring the coefficient of viscosity. These instruments are known as viscometers. The first class, rotational viscometers, is the most important group of viscometric instruments. A container having a fluid therein is rotated around a stationary object, typically a coaxial cylinder that is suspended from a torsion wire. The deflection of the wire produced on a calibrated scale is proportional to the viscous drag.
In the second class of viscometers, a heavy object is allowed to fall freely through the viscous liquid, accelerating initially before it reaches a steady velocity known as the terminal velocity. This velocity, or conversely the time it takes for the object to fall through a predetermined distance, is measured and compared to other fluids with known viscosities. The Laray drop rod viscometer, in which a metal rod falls through an annular space filled with a liquid, is within this class.
The final class of viscometers measures the amount of time it takes for a predetermined volume of liquid to flow through a vertical capillary tube or an orifice. The term "Saybolt seconds", for example, refers to the time of efflux in a Saybolt viscometer.
Each of the above devices suffers from several disadvantages. First of all, those instruments require a relatively large amount of the fluid in order to measure viscosity. Secondly, they cannot detect minute changes in viscosity, which is especially useful in determining the freeze-point of jet fuels and such. Finally, the above viscometers do not allow for localized temperature fluctuations within the subject fluid. As discussed above, viscosity is critically dependent on temperature, and in some cases these fluctuations may result in false readings.
Applicants' invention is drawn to a specialized viscometer overcoming the above limitations through the use of piezoelectric technology. In the past, piezoelectric ceramics have been commonly used as receivers or drivers in electromechanical applications. A good descriptive article entitled "Piezoelectric Ceramics" by Eric A. Kolm, et al. published in Mechanical Engineering, February 1984, page 43, explains the operation of piezoelectric devices. However, it has never in any way been suggested that a piezoelectric type of device may be used to determine viscosity of a fluid. It would, therefore, be desirable and advantageous to devise a piezoelectric viscometer overcoming the above limitations.